The Perception of Speech Under Adverse Conditions

Speech is the primary vehicle of human social interaction. In everyday life, speech communication occurs under an enormous range of different environmental conditions. The demands placed on the process of speech communication are great, but nonetheless it is generally successful. Powerful selection pressures have operated to maximize its effectiveness. The adaptability of speech is illustrated most clearly in its resistance to distortion. In transit from speaker to listener, speech signals are often altered by background noise and other interfering signals, such as reverberation, as well as by imperfections of the frequency or temporal response of the communication channel. Adaptations for robust speech transmission include adjustments in articulation to offset the deleterious effects of noise and interference (Lombard 1911; Lane and Tranel 1971); efficient acousticphonetic coupling, which allows evidence of linguistic units to be conveyed in parallel (Hockett 1955; Liberman et al. 1967; Greenberg 1996; see Diehl and Lindblom, Chapter 3); and specializations of auditory perception and selective attention (Darwin and Carlyon 1995). Speech is a highly efficient and robust medium for conveying information under adverse conditions because it combines strategic forms of redundancy to minimize the loss of information. Coker and Umeda (1974, p. 349) define redundancy as “any characteristic of the language that forces spoken messages to have, on average, more basic elements per message, or more cues per basic element, than the barest minimum [necessary for conveying the linguistic message].” This definition does not address the function of redundancy in speech communication, however. Coker and Umeda note that “redundancy can be used effectively; or it can be squandered on uneven repetition of certain data, leaving other crucial items very vulnerable to noise. . . . But more likely, if a redundancy is a property of a language and has to be learned, then it has a purpose.” Coker and Umeda conclude that the purpose of redundancy in speech communication is to provide a basis for error correction and resistance to noise.